Drying of food
Practical Action
The interval between weighings can be increased when the changes in weight start to become
less. You should also make a note of the wet and dry bulb temperatures of the air inside the
dryer and the air outside. Plot the results on a graph (see Figure 3). You will see the two distinct
phases of drying - the 'constant' and ‘falling’ rate periods. During the constant rate of drying the
surface of the food remains wet which means it can easily be spoiled by moulds and bacteria.
During the falling rate period, the surface is dry and the risk of spoilage is much smaller. For
the best quality dried foods, it is essential to reach the falling rate period as quickly as possible
as this minimises the potential for spoilage. However, it is also important not to reach this point
too quickly because a process known as case hardening may take place (see the box below).
The information collected from a drying rate experiment can be plotted in a different and more
useful way by plotting the moisture content against the change of moisture with time (the rate of
drying). This produces a type of graph that is shown in figure 4. Drying rate is obtained by
calculating the drying rate for each 10 minute period as follows:
Drying rate =
initial weight - final weight
time interval (eg 10 minutes)
The moisture content of both the fresh food and the final dried food can be found by weighing
the food, heating at 100°C in an oven for 24 hours and reweighing. The moisture content is
found as follows:
Moisture content (%) = initial weight - final weight x 100
initial weight
Other values of moisture content during the drying period can be found by relating these two
results to the weights of food recorded during the drying experiment and applying similar factors
to intermediate weights. Figure 4 gives two important pieces of information:
1 The actual drying rate during the
constant rate period which shows how
efficient the dryer is.
2 The final moisture content of the dried
food which shows whether it will be
stable during storage.
A typical drying rate for a solar dryer would be
0.25kg/hr. This depends on the design of the
dryer and the climate. Artificial dryers dry at a
faster rate. Typical values are 10-15 kg/hr. If the
drying rate is lower than this, the air temperature
or speed is too low and/or the RH is too high.
This can be checked by the temperature
measurements made during the experiment and
by using the psychrometric chart. Normally the
air in the dryer should be 10-15ºC above room
temperature in solar dryers and 60-70ºC in
artificial dryers. The RH of air entering the dryer
will vary according to local conditions, but should
ideally be below about 60% RH.
Figure 4: Drying rate/moisture content
5